Resistors are commonly used in semiconductor integrated circuits. Resistors are used, for example, in analog and in mixed mode analog and digital circuits. Resistors are also used in input and output circuits as input and output resistors.
In integrated circuits formed on silicon-on-insulator substrates, a resistor may be formed in a portion of a single crystal silicon layer. Such a single crystal resistor has high stability and low noise, compared with conventional polycrystalline resistor structures. Resistors should also have low parasitic capacitance. Resistors formed on silicon-on-insulator substrates have extremely low parasitic capacitance due to full dielectric isolation and the insulating substrate.
In resistors formed on silicon-on-insulator substrates, the resistor body is usually formed below a silicon oxide layer, which underlies a polycrystalline silicon layer. The polycrystalline silicon layer is usually connected to one of the two terminals of the resistor. With complementary metal-oxide-semiconductor (CMOS) technology scaling, the thickness of the silicon oxide layer is progressively reduced. As the thickness of silicon oxide layer is reduced, leakage current between the polycrystalline silicon layer and the resistor body increases. The increased leakage current results in increased noise.
In addition, resistors are sometimes used as part of an input protection circuit to provide protection of the circuit against electrostatic discharge (ESD) events. In this case, the resistor is used to both attenuate the ESD voltage and also to absorb ESD energy. Large voltages in the order of thousands of volts may appear across the two terminals of the resistor used for ESD applications. Since the polycrystalline silicon layer and the resistor body are connected to the two terminals of the resistor, the silicon oxide layer between the polycrystalline silicon layer and the resistor body may potentially breakdown.